1. Field of the Invention
The present invention relates to a servo valve type timer for a fuel injection pump, and particularly to a servo valve type timer having a timer piston for adjusting a fuel injection timing, the timer piston being equipped with a servo valve.
2. Description of Related Art
There has been hitherto known a servo valve type timer for a fuel injection pump (as disclosed in Japanese Laid-open Patent Application No. Hei-3-275948). This servo valve type timer includes a timer cylinder, a timer spring which is provided in a low-pressure chamber of the timer cylinder, a timer piston which is urged by the timer spring, and a servo valve which is disposed at the low-pressure chamber side of the timer piston and is urged by a servo valve spring provided in the low-pressure chamber. In this servo valve type timer, by pushing down the timer piston, a fuel injection timing is adjusted in accordance with the pressure of a pump chamber, which is introduced into a high-pressure chamber of the timer cylinder.
In this type of timer, the servo valve is designed so that one pressure receiving face thereof is disposed to face the low-pressure chamber and the other pressure receiving face thereof is disposed to face the pump chamber. Hitherto, it has been a general way for adjustment of the fuel injection timing that the servo valve is first shifted to a spark advance side (in a spark advance direction) or a spark delay side (in a spark delay direction), and then the timer piston is shifted to the spark advance side or the delay side by controlling the fuel pressure around the pressure receiving face of the servo valve which faces the pump chamber.
However, it is difficult to greatly vary the pressure in the pump chamber because the pump chamber is designed in large volume. Therefore, in the conventional system of controlling the pressure of the pump chamber side, it is impossible to greatly vary the fuel pressure around the pressure receiving face of the servo valve, and thus a controllable range of the fuel injection timing is limited to a small range.